Current transformer



Aug. 21 19 4- AfBoYAJIAN ET AL 1,971,207

CURRENT TRANSFORMER Filed June 27 1928 Fi gJ.

Inventor's Aham Boya jLan, GuglLelmo CamLLLL,

Their Attorney.

Patented Aug. 21, 1934 UNITED STATES PATENT OFFICE CURRENT TRANSFORMER Application June 27, 1928, Serial No. 288,774

'1 Claims. or. 171-435) Our invention relates to current transformers and the general object of the invention is to provide an improved transformer of this type with accurate ratio and phase angle characteristics. In the usual current transformer, the number of secondary ampere turns is smaller than the number of primary ampere turns and there is an angular phase displacement between the primary and secondary currents because some of the energy of the primary current is used directly to supply hysteresis and eddy current losses in the core. The main object of the present invention is to provide a simple arrangement whereby the core losses of a current transformer may be supplied without substantially disturbing the accuracy of its ratio and phase angle characteristics.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Fig. 1 shows a current transformer arranged in accordance with the invention; Fig. 2 shows the same arrangement shown in Fig. l with the addition of a testing device for indicating proper adjustment of the circuits, and Figs. 3 and 4 show modified forms of the invention.

Like reference characters indicate similar parts in the difierent figures of the drawing.

The particular arrangement of the invention shown in Fig. 1 includes a single turn primary winding 10 surrounded by a magnetic core 11 having a secondary winding 12 and an auxiliary or corrective winding 13. A main load 14 and an auxiliary load 15 are connected in series to the secondary winding 12 and the auxiliary load 15 is connected directly to the auxiliary winding 13. The voltage of the secondary winding 12 induces a current which flows through the main load 14 and also in two parallel paths through the auxiliary load 15 and the auxiliary winding 13.

Part of the voltage of the secondary winding 12 is impressed across the main load 14 and the remainder across the common terminals 16 of the auxiliary load 15 and the auxiliary winding 13. The phase angle of the voltage across the terminals 16 and the relative proportions in which the secondary voltage will divide between the terminals 16 and the terminals of the main load 14 depend upon the electrical characteristics of the two windings 12 and 13 and the two loads 14 and 15. The auxiliary load 15 is adjustable to give any desired value of reactive resistance. Now it is obvious that the phase angle and value of the voltage between the terminals 16 may be adjusted and changed as desired by properly adequal.

justing the reactance and resistance of the auxiliary load 15. It is further obvious that when the vectorial value of the voltage between the terminals 16 is properly adjusted it will force through the auxiliary Winding 13 a current having just the right value and phase angle to supply the exact amount of energy necessary for the core losses. The core is excited entirely by the current in the auxiliary or corrective winding 13 so there is no phase angle displacement between the currents in the primary and secondary windings and the secondary current has the full theoretical value determined by the value of the primary current and the ratio between the primary and secondary turns. Whether the nature of the auxiliary load 15 should be such as to produce a leading or a lagging effect will depend upon the electrical characteristics of the other parts of the device but for purposes of illustration an inductive load to produce a lagging effect is shown in the drawing. If the correction of the ratio and phase angle errors of the transformer requires that the auxiliary load 15 produce a lead- Gil ing rather than a lagging effect, then it is of course obvious that the reactive part of the auxiliary load 15 be a condenser, and preferably a variable condenser, instead of an inductance as shown in the drawing.

The arrangement shown in Fig. 2 is like that shown in Fig. 1 but with the addition by means for indicating when the auxiliary load 15 is properly adjusted. This indicating or testing means includes an auxiliary magnetic core 17 surrounding the primary winding 10 and having two windings l8 and 19. The winding 18 is in series with the main secondary winding 12 and has the same number of turns so that the ampere turns in these two windings are always exactly The winding 19.is connected to a sensi tive detector 20 such as a telephone receiver. When the auxiliary load 15 is adjusted so that the main core 11 is excited entirely by the current in the auxiliary or corrective winding 13, the ampere turns in the windings l0 and 18 must also be exactly equal and balanced. Under these conditions, the auxiliary core 17 is not excited so that no voltage is induced in the winding 19 and the detector 20 is not afiected.

In the modified form of the invention shown in Fig. 3, the two windings 12 and 13 are arranged like the single winding of an autotransformer with the main and auxiliary loads14 and 15 connected in series between its end terminals. The connection between the two loads 14 and 15 is connected to the common terminal of the two windings 12 and 13 or to what may be considered as an intermediate tap of the complete winding considered as an autotransformer winding. The auxiliary load is thus connected to the auxiliary winding 13. The value and phase angle of the current in the auxiliary winding 13 may be adjusted by varying the reactive load 15 and this will affect the value of the current in the main secondary winding 12 and main load 14. The auxiliary load 15 may thus be adjusted until the secondary current in the load 14 has the exact theoretical value determined by the value of the primary current and the ratio between the primary and secondary turns and there is no phase angle displacement between the primary and secondary currents.

In the further modification of the invention indicated in Fig. 4, the windings 12 and 13 are arranged as in Fig. 3 like the two sections of an autotransformer winding with the main load 14 connected across its end terminals. The auxiliary reactive load 15 is connected across the terminals of the auxiliary winding 13. It will be seen that the two loads 14 and 15 are connected in series with the secondary winding 12 and the auxiliary load connected to the auxiliary winding 13. The phase angle and value of the current in the winding 13 may be varied by adjusting the reactive load 15. This will affect the secondary current in the main load 14 and this secondary current may thus be given a value equal to the theoretical value determined by the value of the primary current and the ratio between the primary and secondary turns and may be given a phase angle exactly agreeing with that of the primary current.

The invention has been explained bydescrib. ing and illustrating different embodiments thereof and it will be apparent that further changes may be made without departing from the scope of the invention as defined in the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:--

1. A current transformer including a magnetic core having primary, secondary and auxiliary windings, a main load, and an auxiliary load having reactive impedance, said two loads being connected in series to said secondary winding, and said auxiliary load being connected directly to said auxiliary winding, the impedance of the auxiliary load being proportioned to maintain in said auxiliary winding a current of vectorial value substantially to supply the transformer losses between said primary and secondary windings.

2. A current transformer including a magnetic core having primary, secondary and auxiliary windings, a main load, an adjustable auxiliary load having reactive impedance, said two loads being connected in series to said secondary winding, and said auxiliary load being connected directly to said auxiliary winding, and means for adjusting the impedance of said auxiliary load to maintain in said auxiliary winding a current of vectorial value substantially to supply the transformer losses between said primary and secondary windings.

3. A current transformer including a magnetic core having primary, secondary and auxiliary windings, a main load, an auxiliary load having a reactive resistance, said two loads being connected in series to said secondary winding, and said auxiliary load being connected directly to said auxiliary winding, and means for independently adjusting the reactance and resistance of said auxiliary load to maintain in said auxiliary winding a current of vectorial value substantially to supply the transformer losses between said primary and secondary windings.

4. The combination with a current transformer, of a corrective winding for said transformer, and means for energizing said corrective winding comprising a conductor having current therein out of phase with the current in the primary of said current transformer.

5. The combination with a current transformer, of a corrective winding for said transformer, and means for energizing said corrective winding comprising a conductor, and a source supplying voltage to said conductor out of phase with the voltage impressed on the primary winding of said current transformer but having a definite phase relation therewith.

6. The combination in a current transformer, of a magnetic core member providing a flux path, a primary winding arranged for connection in a primary circuit and surrounding said flux path, a secondary winding arranged for connection in a secondary circuit and surrounding said flux path, a corrective winding surrounding said flux path, and means for energizing said corrective winding comprising a conductor having current therein out of phase with both primary and secondary winding currents.

'7. The combination in a current transformer, of a magnetic core member providing a flux path,

a primary winding arranged for connection in a.

primary circuit and surrounding said flux path, a secondary winding arranged for connection in a secondary circuit and surrounding said flux path, a corrective winding surrounding said flux path, and means connected to one of said circuits for energizing said corrective winding by current out of phase with the current in the circuit to which the energizing means is connected.

ARAM BOYAJIAN.

GUGLIELMO CAMILLI. 

